1. Field of the Invention
The present invention relates to an AC voltage control circuit for controlling AC voltage in such a manner that voltage can be constantly outputted regardless of fluctuation of inputted AC voltage and in particular to an AC voltage control circuit for use in a power saving device for a discharge lamp.
2. Description of the Prior Art
Illumination apparatuses using light obtained from the discharge of gas filled in the apparatuses after the gas has been electrically discharged, such as fluorescent lamps, mercury lamps, and metal lamps, are generically named as “discharge lamp”. Such discharge lamps generally require predetermined level of voltage such as 120V or 277V so as to initiate the discharge. However, once the discharge is produced and thus the lamps are turned to ON, they can emit light without any difficulty even if voltage lower than the predetermined voltage is applied so as to maintain the discharge.
For example, in the case of a large-scaled signboard employing 100 to 300 sodium-vapor lamps at one time, voltage in the range of about 80 to 90% of initial voltage may be applied to the signboard at the discharge maintaining step. In such a case, a great power saving effect can be achieved.
As an example of power saving devices exhibiting such a power saving effect, one power saving device is known, which first applies 120V (or 277V) to a discharge lamp so that discharge is initiated in the discharge lamp when the discharge lamp is turned ON. It then applies minimum discharge maintaining voltage lower than the discharge initiating voltage to the discharge lamp after the discharge is completed, thereby producing a power saving effect, wherein the minimum discharge maintaining voltage is the minimum voltage applied to the discharge lamp at which the discharge lamp can emit light with a predetermined level of illumination and/or brightness required for humans. FIG. 1 shows one example of such a power saving device of the prior art.
The power saving device 100 for a discharge lamp shown in FIG. 1 has a pair of coils C1, C2, which are wound in different directions, and an insulation member INS interposed between the coils C1, C2, wherein the pair of coils C1, C2 serve as a compound transformer with subtractive polarity and the coils C1, C2 generates inducted voltage. The circuit has first and second switches S1, S2 and the secondary coil is provided with a plurality of taps T1, T2, T3, and T4. As indicated dotted lines, a pair of terminals P1, P2 are connected to an external power source E and another pair of terminals P3, P4 are connected to the discharge lamp L.
At the initial step for lighting the discharge lamp, the first switch S1 is closed and the second switch S2 is opened, in which if the discharge lamp L is lighted by primary voltage applied from the external, the second switch S2 is connected with one of the plural taps T1, T2, T3 and T4 by a timer 14 connected with the first and second switches S1, S2 and the first switch S1 is opened. Thereafter, the induction voltage induced in the coils C1, C2 is transferred to the discharge lamp, thus maintaining the continuous lighting of the discharge lamp, wherein the induction voltage induced in the coils C1, C2 is lower than the primary voltage. Therefore, a power saving effect can be obtained.
On the other hand, such a power saving device for a discharge lamp also requires a voltage regulator which can supply constant and stable output voltage regardless of fluctuation of voltage inputted into the power saving device 100. A conventional automatic voltage regulator (AVR) essentially consists of a multi-tap transformer and a switch. The automatic voltage regulator senses fluctuation of the output voltage and is adapted to render predetermined voltage to be outputted by intermitting the multi-taps of the transformer using a semiconductor switching device, or the like, depending on the fluctuated output voltage sensed by the regulator.
In addition, the conventional power saving device for a discharge lamp shown in FIG. 1 further includes a protection device against abnormal voltage, an over-current cut-off device, or the like for preventing voltage and current flowing in the power saving circuit from being increased over or decreased below a normal value, as well as the above-mentioned automatic voltage regulator. Moreover, as shown in FIG. 1, the conventional power saving device further includes a timer 14 connected to the switches S1, S2, as well as a display device for informing a user of the voltage or current in the power saving device circuit. Eventually, the conventional power saving device 100, as being practically used, is separately provided with numerous additional circuits at the input side and/or output side of the power saving device 100 beyond the basic circuit structure shown in FIG. 1. Therefore, the entire volume of the power saving device and manufacturing costs thereof are increased. Furthermore, because numerous additional circuits are individually provided, inconvenience in maintaining and controlling such a power saving device follows, from a user's standpoint.